In a field-effect transistor, a short-channel effect becomes noticeable, as the field-effect transistor is microfabricated, and in a conventional single-gate transistor, a high-concentration channel impurity is required to suppress the short-channel effect. However, it has been known that the increase in the concentration of the channel impurity causes a reduction in on-state current due to deterioration in a carrier mobility in the channel, an increase in variability of a threshold voltage due to fluctuation in impurity distribution, and an increase in junction leakage current. Therefore, suppressing the short-channel effect without relying on the increased concentration of the channel impurity is needed to enhance performance of the microfabricated transistor.
There has been proposed many types of multi-gate transistors having plural gate electrodes to a channel, as a method of realizing a suppression of a short-channel effect without relying on the increased concentration of the channel impurity. Since the multi-gate transistors control a channel potential by the plural gate electrodes, the controllability of the gate electrodes over the channel potential is stronger than that of a drain electrode, whereby the short-channel effect can be suppressed without increasing the concentration of the channel impurity. A fin field-effect transistor is one of the multi-gate transistors. When a height of a fin channel is increased, a channel width is increased, whereby on-state current can be increased without increasing the footprint of the transistor. Therefore, it is suited for a cell transistor of a high density memory LSI requiring high drive current, for example.
The fin field-effect transistor is classified into a type formed on a bulk semiconductor substrate, and a type formed on an SOI (Silicon On Insulator) substrate. The former type is preferable from the viewpoint of a semiconductor wafer cost, process compatibility with a conventional planar bulk transistor, and suppression of self-heating. The fin field-effect transistor of the former type needs a punch-through stopper under the fin channel region in order to prevent a leakage current that flows between a source and a drain. In the process of the punch-through stopper formation via ion implantation, an impurity is also doped into the channel. Therefore, this transistor entails a problem that the concentration of the channel impurity is increased.